# Design of Electrical Machines Questions and Answers – Pole Design

This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Pole Design”.

1. What are the factors the design of poles of the DC machine depends on?
a) length, breadth, height of the conductors
b) area of cross section of poles
c) area of cross section of poles and height of the poles
d) area of cross section of poles and height of the poles and the design of field windings

Explanation: For designing the poles, first the area of cross section of the poles and the height of the poles should be obtained. Then the field winding design details are also required during the pole design.

2. What is the relationship between flux in the pole body and the useful flux per pole?
a) flux in the pole body is directly proportional to useful flux per pole
b) flux in the pole body is indirectly proportional to the useful flux per pole
c) flux in the pole body is directly proportional to the square of useful flux per pole
d) flux in the pole body is indirectly proportional to the square of useful flux per pole

Explanation: According to the flux in the pole body formula the flux in the pole body is directly proportional to the useful flux per pole. It is also proportional to the leakage coefficient.

3. What is the flux in the pole body, given leakage coefficient = 1.2 and the useful flux per pole is 10 weber?
a) 12 weber
b) 11.2 weber
c) 8.2 weber
d) 20 weber

Explanation: Flux in the pole body = leakage coefficient * useful flux per pole
Flux in the pole body = 1.2 * 10 = 12 weber.

4. What is the meaning of useful flux?
a) the flux which is being created in the machine
b) the flux which can be used
c) the flux which can produce the output
d) the flux that is wasted

Explanation: Total flux is the maximum amount of flux that is being generated by the current flowing in the circuit. Useful flux is nothing but the flux which can produce the output in the machine.

5. What is the range of leakage coefficient for output of 100kW?
a) 1.12-1.25
b) 1.11-1.22
c) 1.10-1.20
d) 1.11-1.15

Explanation: 1.12-1.25 is the leakage coefficient when the output is 50kW. 1.10-1.20 is the leakage coefficient when the output is 200kW.
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6. What is the range of leakage coefficient for output of 1000kW?
a) 1.12-1.25
b) 1.11-1.22
c) 1.09-1.18
d) 1.08-1.16

Explanation: 1.12-1.25 is the leakage coefficient when the output is 50kW. 1.11-1.22 is the leakage coefficient when the output is 100kW. 1.09-1.18 is the leakage coefficient when the output is 500kW.

7. What is the range of the flux density in the pole shrank for laminated poles?
a) 1.1-1.7 Wb per m2
b) 1.2-1.6 Wb per m2
c) 1.3-1.7 Wb per m2
d) 1.2-1.7 Wb per m2

Explanation: The flux density in the pole shrank of laminated poles should have a minimum value of 1.2. The flux density in the pole shrank of the laminated poles should not exceed 1.7 at the same time.

8. What is the formula for the area of the poles shrank of the laminated poles?
a) area of the pole shrank = flux in the pole body * magnetic field
b) area of the pole shrank = flux in the pole body + magnetic field
c) area of the pole shrank = flux in the pole body – magnetic field
d) area of the pole shrank = flux in the pole body / magnetic field

Explanation: For finding out the area of the pole shrank first the flux in the pole body is found out using the product of the leakage coefficient and the useful flux in the pole. Next, the magnetic field is measured and the product gives the area.

9. What should be the length of pole with respect to the length of the armature and what should be the range of the length of pole?
a) length of pole < length of armature, 10-15 m
b) length of pole > length of armature, 10-15 mm
c) length of pole > length of armature, 10-15 cm
d) length of pole < length of armature, 10-15 mm

Explanation: The length of the pole should be very much less than the length of the armature in order to permit the end play and t avoid magnetic centering. It should be in the range of 10-15 mm.

10. The formula for length of pole is L = Total length of armature – (0.001 to 0.005).
a) true
b) false

Explanation: First the total length of armature is calculated. Then the value is subtracted by 10-15 mm in order to obtain the length of the pole.

11. What is the formula for the width of pole of DC machines?
a) width of pole body = area of the pole * length of the pole
b) width of pole body = area of the pole + length of the pole
c) width of pole body = area of the pole – length of the pole
d) width of pole body = area of the pole / length of the pole

Explanation: For obtaining the width of the pole, the area of the pole is first obtained. Then the length of the pole is calculated. The ratio of area of the pole to the length of the pole gives the width.

12. Height of the pole depends on the mmf to be provided on the pole at full load.
a) true
b) false

Explanation: The height of the pole totally depends on the mmf provided to the poles. The mmf provided at full load is only taken into consideration for the height measurement.

13. How is the mmf required at full load obtained for the calculation of height of poles?
a) using closed circuit characteristics
b) using open circuit characteristics
c) using formula
d) using equivalent circuit

Explanation: The mmf at full load is calculated using the magnetization curve. The open circuit characteristics are obtained which help in finding out the mmf at full load.

14. How should the field mmf be with respect to armature mmf to reduce the armature reaction?
a) armature mmf > field mmf
b) armature mmf >= field mmf
c) armature mmf < field mmf
d) armature mmf = field mmf

Explanation: To reduce the armature reaction, the field system should be designed such that the field mmf should be dominant over the armature mmf. If the armature mmf becomes low, the armature reaction reduces.

15. What should be the range of the field mmf to armature mmf ratio at full load?
a) 1.0-1.2
b) 1.1-1.3
c) 1.3-1.5
d) 1.1-1.25

Explanation: The minimum value of the ratio should be atleast 1.1. The maximum value of the ratio should be not greater than 1.25.

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